1. Field of the Invention
The present invention relates to an antenna selection diversity reception apparatus wherein switching of branches is executed for each time division multiplexing (to be called hereinafter as TDM) burst of digital signals in the digital vehicle communication system.
2. Prior Art
FIG. 1 is a block diagram of the antenna switching diversity reception apparatus according to a prior art as described for example in the literature titled "Mobile Telephone" (compiled by Kuwabara published in 1985 by the Institute of Electronics, Information and Communication Engineers). In FIG. 1, numerals 1a, 1b designate antennas adapted to receive the digital modulation signals in the digital vehicle communication system, and numeral 2 is an antenna switching system for selecting and switching the respective antennas 1a, 1b. Numeral 3 designates a reception system to which the signals received by the antenna 1a or 1b which has been selected by the antenna switching system 2 and in which the signals are subject to high frequency amplification, frequency conversion, intermediate frequency amplification, and so forth. Numeral 4 designates a demodulation system for demodulating the transmission data from the signals output from the reception system 3.
Numeral 5 designates an antenna selection decision system adapted to generate antenna selection signals for controlling the antenna switching system 2 in accordance with the signals from the reception system 3. In the antenna selection decision system 5, numeral 51 designates a level detector adapted to detect the level of the signals which are sent to the demodulation system 4 from the reception system 3, and 52 designates an antenna selection decision device adapted to compare the level detected by the level detector 51 with a reference level which has been provided in advance, so as to generate the antenna selection signals for controlling selection of either of the antennas 1a, 1b and provide these signals to the antenna switching system 2.
Operation of the diversity reception apparatus as described above will be explained. The antennas are switched according to this system when the reception level falls below the reference level. More specifically, in such a case as when the antenna 1a is connected to the reception system 3 by means of the antenna switching system 2, the reception system 3 amplifies at a high frequency the signal received by the antenna 1a, converts this signal to an intermediate frequency signal, amplifies this intermediate frequency signal, and then provides it to the demodulation system 4. The demodulation system 4, in turn, detects the intermediate frequency signal, demodulates the transmission data and outputs this data as output data. The signal which has been output from the reception system 3 is also provided to the antenna selection decision system 5. The antenna selection decision system 5 will detects its level and decides the antenna for reception. Level detector 51 detects the level of the signal provided by reception system 3. The antenna selection decision device 52 is adapted to compare the level of the signal which has been detected by the level detector 51 with the reference level provided in advance for generating the antenna selection signal which is provided to the antenna switching system 2. The antenna switching system 2 is controlled by this antenna selection signal and adapted to select either of the antennas 1a or 1b which has been designated, and to send the resultant reception signal to the reception system 3.
It is to be understood here that there are both a "switch and stay" method (to be called as SS method) and a "switch and examine" method (to be called as SE method) applicable to the antenna switching operation. FIG. 2A and FIG. 2B, show the changes in the received wave envelope levels for each methods mentioned above. As shown in FIG. 2A, the SS method keeps the reception antenna for a certain time even if the level of the signal after the antenna has been switched is lower than the reference level. On the other hand, according to the SE method as shown in FIG. 2B, when the level of the signal after the antenna has been switched is lower than the reference level, the operation of switching the antennas between 1a and 1b is repeated at a predetermined switching rate until the level of the signal received by either of the antenna 1a or 1b reaches greater than the reference level. This is described in detail in such literature as the one titled "Principle of Vehicle Communication" (compiled by Okumura and Shinji, published in 1986 by the Institute of Electronics, Information and Communication Engineers).
Though the above-described methods of antenna switching are suitable for miniaturization and economization of the relevant circuit, they are unsuitable that antenna switching before signal detection causes unavoidable noise arising at the time of switching. In the case of an analog transmission system of frequency modulation (FM) or digital transmission system of frequency division multiple access (FDMA), such reception characteristics as expressed by the envelope level shown by thick solid lines in FIG. 2A and FIG. 2B may be provided as far as this switching noise remains at an acceptable level. However, according to TDM digital transmission system, since communication is conducted in the units of a TDM burst, transmission of TDM burst to be received is finished before operation of the receiver is stabilized for reception when the antenna 1a or 1b is switched in the course of a TDM burst. In view of this problem, there is proposed such a method of receiving one whole TDM burst by using the selected antenna 1a or 1b in such a manner that the antenna 1a or 1b is not switched during reception of a TDM burst, where the level of the signals for the TDM burst allocated to another user immediately preceding user's own burst is detected so as to select the antenna 1a or 1b which has a higher level.
The above-mentioned method which is suitable for TDM transmission method enables the identical antenna 1a or 1b to receive a signal entirely through one TDM burst and is capable of demodulating the signal. In this sense, this method is known as an antenna selection diversity reception method. This sort of antenna selection type diversity reception method is described in detail in such literature as an article titled "Antenna Selection Diversity Reception Method of Digital Vehicle Communication" (authored by Akaiwa, a Collection of Articles in connection with Spring National Session 1989 of the Institute of Electronics, Information and Communication Engineers, Vol. 2, p. 360). According to such an antenna selection diversity reception method, two preamble sections 6 or P.sub.1, P.sub.2 and an information section 7 are constituted in a frame 8 of a given length as shown in FIG. 3 and thus transmitted. Switching of the antennas 1a, 1b is executed only in the zones of the relevant preamble sections 6. Specifically, according to the example shown in FIG. 3, the level of the signal received by the antenna 1a is measured in the zone P.sub.1 in the preamble section 6 while the level of the signal received by the antenna 1b is measured in the zone P.sub.2. Thus the receptive levels are compared. As the result, the antenna 1a or 1b which has a higher level is selected and the information section 7 of the frame 8 received by the selected antenna 1a or 1b is provided to the reception system 3 for further reception and demodulation.
Since the conventional antenna selection diversity reception apparatus is constituted as described above, two preamble sections 6, i.e. P.sub.1 and P.sub.2 having the same function are required in one TDM burst, which inevitably results in a reduction in the efficiency of information transmission. On the other hand, a post detection deversity reception is frequently employed for automobile-installed vehicle communication apparatus. Problems also arise such as the provision of the plural preamble sections 6 having the identical function is not preferable in view of the system constitution only for the antenna selection diversity apparatus which is mainly used for a portable set. In order to solve this problem, it is conceivable to provide such a method wherein measurement of the level of the signals for selecting the antennas is made by measuring the reception level of the immediately preceding TDM burst (the term "the immediately preceding TDM burst" is hereinafter used to mean the TDM burst which has been allocated to another user, such TDM burst immediately preceding TDM burst allocated to present user) by switching the antennas 1a and 1b in specific cycles so as to select the antenna 1a or 1b which has a higher reception level, whereby TDM burst of the user's own channel received by the selected antenna is received and demodulated. However, in the case of the digital mobile communication system, when the transmission power from the base station to the mobile station is restricted in order to reduce interference between the channels which uses the same frequency and overlap each other in some time region in the system, it is possible that the level of the average reception signals of the immediately preceding TDM burst will be very low. In such an instance, the effect of measuring the reception level cannot be expected. When no immediately preceding TDM burst exists due to the existence of a vacant channel, measurement of the level in itself will be impossible due to a lack of transmission from the base station.